Is there a word or simple phrase to describe the relationship between yourself and a step-parent's ex-spouse? Particularly, I'm seeking to address the case when the ex-spouse is a parent to your step-sibling.

Example: Bob’s mother re-married to Alice’s father. How should one appropriately refer to the relation between Bob, and Alice’s mother?

I’m trying to find something that’s appropriate for a relationship closer than “Bob’s step-sister’s mother” or “Bob’s step-father’s ex-wife” but not quite so close as to just call her “Bob’s mom”.

Interesting question. I've always been vexed at the similar issue of how to refer to my brother's wife's sister. She's my sister-in-law's sister, or my brother's sister-in-law, but to me she's what?
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SnubianMay 10 '11 at 3:33

Even if you can find a good word or phrase, prepare to be misunderstood. In a recent answer, I referred to my stepmother's father as my "step-grandfather"; at least one commenter changed this to "stepfather".
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MT_HeadJun 22 '11 at 23:35

According to Yiddish Dictionary Online, this word is mishpokhe and means "family" or "kin". Thanks for the suggestion!
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IsziMay 10 '11 at 0:29

I'm used to mishpacha (no 'b'; it's a 'p') describing the down-over-up of a couple's parents. My parents are my husband's parents' mishpacha and vice-versa. (This case is not served by the "in-law" form without additional modifiers.) I don't know how common its usage is with step-relations, exes, and weird cousin Billy that no one wants to talk about.
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Monica CellioSep 4 '12 at 15:04

I think I would call her "my stepsister's mother", since that is the relationship that is important to you. Also, anyone who takes a moment to think would figure out that you are not talking about your own mother OR stepmother, since you would have said so in the first place.